hey everybody, new to the forum and need a little advice w/ some speakers ive already made and have used for awhile. they are 7.5 - 8 cu. ft. cubes loaded w/ eminence kappa 15 lfa. they have two 1.5x 10x 4 inlong slot ports at the bottom of the cab. they dont seem to have alot of output. sound bassier out the back. using them in a classic rock band under some sp5 peaveys crossed at about 100hz and limited w/ low cuts at 30hz. using a peavey cs 3000 to power them. have looked around alot on the web for proper port tuning. dont need anything below 40hz. any info,advice on port size greatly appreciated. have a great day

A 1.5" x 10" vent 4" long will tune 7.5-8 ft^3 net in the ~25-26 Hz range or a little lower if it's a shelf vent, so you're going to need a larger vent to tune it at ~40 Hz at low compression. Assuming a 3/4" baffle thickness vent, it needs to be a 6" diameter hole or its square/rectangular equivalent at minimum.

Based on driver specs and 400 W rating though, four 5" dia. vents ~13.88" long depending on the cab's net Vb seems the minimum, but these will reduce the cab's net Vb enough to probably require them to be longer, so ideally you'll have to fine tune them. Me, I'd just up the high pass to ensure they don't unload below tuning at high power.

Note that in a sim, you run the risk of bottoming them out at rated power in the 50-70 Hz range in a typical prosound half space (floor) loading............

GM

__________________Loud is Beautiful if it's Clean! As always though, the usual disclaimers apply to this post's contents.

thanks for taking time to answer. can the existing ports be modified to tune it to 40hz. shortening them? when you said you would up the high pass, do you mean you wouldnt try to adjust the port. ive only got room to put 3in ports in the frt. once again thanks for your time. is the 3/4 in. baffle considered a 3/4 port.

Assuming you can shorten your rectangular vent to 3/4", then expect a 33-35 Hz tuning.

No, I'm saying that if you increase the tuning, then the driver(s) need to be protected to a higher frequency.

Yes, a cutout in a baffle is a vent and has a physical length of however thick it is. This is usually called a bass reflex vent.

Anyway, basically, assuming your existing vent is reduced to baffle thickness, you need to add more cutouts to ~ double the existing vent area to get a ~40 Hz tuning, but it may not be able to handle the driver's output at rated power without a lot of port compression which is no big deal in a HIFI/HT app, but usually is in a prosound app. Only one way to know for sure though whether it's a big deal to you in your app.

GM

__________________Loud is Beautiful if it's Clean! As always though, the usual disclaimers apply to this post's contents.

Gm did you note the number of ports that jdrum posted - You seem to be assuming one port with the lengths you are quoting.

Jrum Check the speakers for phasing. Use a battery on the leads and ensure both speakers move in the same direction. Then check the amplifier, make sure that it is not set up for bridge mode for instance!
Seems obvious but I had all sorts of problems in the past with a borrowed pair of bins that had been incorrectly wired. I could never get them to sound right! Do whatever it takes to ensure the phasing is correct, if need be reverse the speaker lead polarity to one speaker and compare the bass output.

Nope, guess I had another 'senior moment' , so thanks for pointing it out!

Right now then, they're tuned in the 33-34 Hz range and reducing the two existing vents to 3/4" long yields around 40-42 Hz depending on actual net Vb, but to handle 400 W at low distortion requires two more vents same as the existing ones and maybe all four will need to be lengthened an inch or more to get the desired tuning.

GM

__________________Loud is Beautiful if it's Clean! As always though, the usual disclaimers apply to this post's contents.